In answer to sbhoa, practising an improvisation and preparing/memorizing a passage and calling it an improvisation are two somewhat different things.
Honstly, I do know what I'm talking about; I'm a working jazz musician myself and I know very well when I play something I've played before, and it's not improvisation!!! At every gig I do my utmost best to play something I've never played before, on EVERY SINGLE NUMBER, EVERY SINGLE TIME. Otherwise what's the point and who am I kidding?
So suppose you have to improvise over a chord progression of C Am Dm G7 and you've been working on using notes from 2 scales: C and G7 bebop, then you spend your time practising those scales and playing riffs, runs, licks and arpeggios of those, developing your ideas more and more each time you play.
Say one day you play something really inventive and unique - do you think, hey that was great! I must play that in my exam? Or do you just play and play until you are so fluent in those scales and know the music so well that when the time comes you can always pull a pretty good rabbit out of the hat?
Look, for a second opinion I asked my son's sax teacher about it today. My son is working towards Grade 5 jazz sax, Guildhall syllabus. I asked him if a prepared "impro" would be acceptable in an exam and he looked at me as if I was completely mad. He told me this would completely ruin the purpose of that part of the exam, which is to test for your improvising skills, and he pointed out that being able to improvise is pretty much the main thing in jazz.
But then he's a working jazz musician himself so he's hardly likely to think a pre-prepared solo acceptable in an exam, is he? And nor should he be.
Perhpas there's a problem here with classically orientated teachers learning a bit of jazz, up to Grade 5 for example, and then teaching the jazz syllabus with all their classical habits deeply engrained and unable to pass on a real sense of what jazz is actually about, which is, as my son's sax teacher re-iterated tonight, primarily IMPROVISING ON THE SPOT.
The confusion here seems to be between practising and preparing a solo - these are two different things! Practise playing through the changes - you need to do this for sure! But if you want to play jazz and do something genuine in the exam, IMPROVISE in the exam!
Otherwise what are you doing it for? To add some more Grade notches to your bedpost, or to explore and actually learn to play some jazz?
You sound like you'll be fine, Margaret, if you were happy to improvise in your Grade 8 practical musicianship, but the very best of luck anyway!